Hepatitis B is one of the world's most prevalent diseases. Although most individuals seem to resolve the infection following acute symptoms, approximately 30% of cases become chronic. According to current estimates, 350-400 million people worldwide have chronic hepatitis B, leading to 500,000-1,000,000 deaths per year due largely to the development of hepatocellular carcinoma, cirrhosis, and other complications. Despite the availability of an effective vaccine, immunoglobulin therapy, interferon, and antiviral drugs, hepatitis B remains a major global health problem.
The causative agent is hepatitis B virus (HBV), a small DNA virus that is considered to be the prototypical member of the hepadnaviridae. HBV is an enveloped virus with an unusual mode of replication, centering on the establishment of a covalently closed circular DNA (cccDNA) copy of its genome in the host cell nucleus. This episomal form is established from conversion of the partially double stranded circular DNA (relaxed circular, or rcDNA) genome upon initial infection, and functions as the template for all HBV mRNAs. Unlike the mechanisms of most other DNA viruses, HBV cccDNA replicates through retrotranscription of a 1.1 genome unit-length RNA copy (pregenomic, or pgRNA) which is originally transcribed from the cccDNA template, and which is acted upon by a virus-encoded polymerase to yield progeny rcDNA. HBV DNA synthesis is coupled to assembly of its capsid, and most copies of the encapsidated genome then efficiently associate with the envelope proteins for virion assembly and secretion; a minority of these genomes are shunted to the nucleus where they are converted to cccDNA, thus amplifying levels of the episome.
As the only enzyme encoded by HBV, the polymerase has been well-exploited as a target for antiviral drug development, with four nucleoside-analogous polymerase inhibitors already FDA-approved, and others in development. Mutations in the primary sequence of the polymerase that confer resistance to lamivudine and adefovir have been identified clinically, and underlie a rebound of serum virus titers that 70% of treated patients experience within three years of starting lamivudine therapy. Although resistance to telbivudine, adefovir and entecavir occurs more rarely, it has been recorded. α-Interferon is the other major therapy available for hepatitis B, but is limited by poor long-term response and debilitating side effects. Hence, there is certainly a medical need for treatments with improved characteristics, and for a diversity of approaches in developing HBV therapies.
Aside from being a critical structural component of the virion, the HBV envelope is a major factor in the disease process. In chronically infected individuals, serum levels of HBV surface antigen (HBsAg) can be as high as 400 μg/ml, driven by the propensity for infected cells to secrete non-infectious subviral particles at levels far in excess of infectious (Dane) particles. HBsAg comprises the principal antigenic determinant in HBV infection and is composed of the small, middle and large surface antigens (S, M, and L, respectively). These proteins are produced from a single open reading frame as three separate N-glycosylated polypeptides through utilization of alternative transcriptional start sites (for L and M/S mRNAs) and initiation codons (for L, M and S).
Although the viral polymerase and HBsAg perform very different functions, both are essential proteins for the virus to complete its life cycle and be infectious. That is, HBV lacking HBsAg is completely defective and cannot infect or cause infection. HBsAg is needed to protect the virus nucleocapsid, to begin the infectious cycle, and to mediate morphogenesis and secretion of newly forming virus from the infected cell.
People who are chronically infected with HBV are usually characterized by readily detectable levels of circulating antibody specific to the viral capsid (HBc), with little, if any detectable levels of antibody to HBsAg. There is some evidence that chronic carriers do produce antibodies to HBsAg, but these antibodies are complexed with the circulating HBsAg, which can be present in milligram per milliliter amounts in a chronic carrier's circulation.
Reducing the amount of circulating levels of HBsAg might permit whatever anti-HBsAg is present to gain a foothold and enable the antibody to manage the infection. Moreover, even if nucleocapsids, free of HBsAg, were to be expressed or secreted in to the circulation, perhaps as a result of cell death, the high levels of anti-HBc would be expected to quickly complex with them and result in their clearance.
A study of duck hepatitis B virus (DHBV) has indicated that the presence of subviral particles in a culture of infected hepatocytes may have a transactivating function on viral genomic replication. In addition, a long-held tenet of HBV biology is that this circulating surface antigen functions to suppress virus-specific immune response. In chronic woodchuck hepatitis virus (WHV) infection, a reduction of antigenemia through clevudine treatment resulted in a positive response to vaccination indicating that circulating antigen may indeed suppress the immune response. Furthermore, the scarcity of virus-specific cytotoxic T lymphocytes (CTLs) that is a hallmark of chronic WHV and HBV infection may be due to repression of MHC I presentation by intracellular expression of L and M in infected hepatocytes. Existing FDA-approved therapies do not significantly affect HBsAg levels in the serum.